Systems Homeostasis Lecture Notes 2020 PDF

systems homeostasis Alexander Burton Adjunct Fellow School of Medicine Western Sydney University Learning Outcomes Describe the principles of homeostasis and how the internal environment is maintained essentially constant through negative feedback control Recognise that homeostasis requires detector...

systems homeostasis Alexander Burton Adjunct Fellow School of Medicine Western Sydney University Learning Outcomes Describe the principles of homeostasis and how the internal environment is maintained essentially constant through negative feedback control Recognise that homeostasis requires detectors to inform the system of the current state, a controller to compare the current state with the required state (set-point) and effectors to bring the current state to the set-point Recognise that many physiological systems rely on negative feedback to maintain a constant internal environment, but that set-points can be changed to accommodate different physiological requirements Homeostasis “homeo” = same “stasis” = stand Maintaining a constant level, a steady state The term homeostasis was coined by American physiologist Walter Bradford Cannon (1871-1945), but the foundations of the principles were laid earlier by French physiologist, Claude Bernard Le mileu interieur (the internal environment) The French physiologist Claude Bernard (1813-1878) stated that: “the living body, though it has need of the surrounding environment, is nevertheless relatively independent of it. This independence… derives from the fact that… the tissues are withdrawn from direct external influences and are protected by a veritable internal environment which is constituted, in particular, by the fluids circulating in the body.” “The constancy of the internal environment is the condition for free and independent life: the mechanism that makes it possible is that which assured the maintenance, within the internal environment, of all the conditions necessary for the life of the elements.” “The constancy of the environment presupposes a perfection of the organism such that external variations are at every instant compensated and brought into balance. In consequence, far from being indifferent to the external world, the higher animal is on the contrary in a close and wise relation with it, so that its equilibrium results from a continuous and delicate compensation established as if the most sensitive of balances.” At the time Claude Bernard was formulating his ideas on maintaining the internal environment constant, there was rapid development of machines that increased productivity during the so-called Industrial Revolution Inventions like steam engines required control systems to maintain a constant output: Scottish engineer James Watt developed the centrifugal governor to allow a constant speed of rotation Homeostasis What do we need to maintain a steady state? We need to know what the current state is We need to know what the current state should be We need to bring the current state to the correct state Homeostatic control: Detector(s) Controller(s) Effector(s) Control is brought about through “negative feedback” An error signal is detected and corrected by the controller Examples of physiological control processes involved in homeostasis: 1. 2. 3. 4. 5. Control of blood pressure Control of body temperature Control of food intake Control of osmolality Control of blood volume Control of blood pressure: Arterial pressure is controlled continuously (beat-beat): Sympathetic vasoconstriction Sympathetic and parasympathetic (cardiac vagal) effects on heart rate and stroke volume The control system (the baroreflex) is a negative feedback loop: an increase in arterial pressure brings about a decrease in arterial pressure (and vice versa), maintaining a constant (set) level The baroreflex is a negative feedback loop Arterial baroreceptors, located in the carotid sinus and aortic arch, monitor arterial pressure the control “centre” is located in the medulla During exercise blood pressure increases but the baroreflex still operates to keep blood pressure essentially constant – the set-point is increased This involves changing the central controller within the medulla – resetting the baroreflex to operate at a higher set-point Likewise, in high blood pressure (hypertension) the baroreflex still operates, but at a higher set-point Control of body temperature: Temperature receptors are located in the skin and in the hypothalamus Loss of heat to the environment is brought about by an increase in sympathetic sudomotor (sweating) activity and a decrease in cutaneous vasoconstrictor activity Gain of heat is brought about by an increase in cutaneous vasoconstriction and increase in activity of the skeletal muscles (shivering) Control of body temperature: Gain of heat is brought about by an increase in cutaneous vasoconstriction and increase in activity of the skeletal muscles (shivering) A pyrogenic fever results in a change in temperature set point, resulting in a perception of being cold Shivering occurs in an attempt to increase the core temperature Once the core temperature has increased beyond the set point sweating commences in an attempt to decrease core temperature control of body temperature Detectors: In some homeostatic systems negative feedback is provided by circulating hormones or chemicallymediated changes in neural activity in an organ In other systems, mechanical signals as well as chemical signals (local and circulating) provide feedback Control of food intake rat brain Control of osmolality Molecular Physiology of Water Balance Knepper, et al. N Engl J Med 2015; 372:1349-1358 Control of blood volume Homeostasis occurs over different time scales Homeostasis is usually controlled within the hypothalamus or brainstem Conclusions: Homeostasis operates on many different organ systems, and on different timescales Homeostasis requires detectors to sense the physiological variable, a controller (usually in the hypothalamus or brainstem), and effectors to cause a change Homeostasis operates through negative feedback: the detected signal is compared to the expected signal (set point) and corrected accordingly set-points can be changed to accommodate different physiological requirements Questions? [email protected]